1. Field of the Invention
The present invention relates to a variable-capacitance element, a variable-capacitance device, and a portable phone including a variable-capacitance device.
2. Related Art
A digital variable-capacitance element manufactured using a micro-machine or MEMS (Micro-Electro-Mechanical Systems) has such merits that a variable range is wide, reduced strain is provided, Q value is large (loss resistance is small), as compared with a variable-capacitance element including a PIN diode. Therefore, the digital variable-capacitance element is suitable for use in a matching circuit for a small-sized wide range antenna in a portable apparatus.
It is known to constitute a digital variable-capacitance device with a wide variable range by using a digital variable-capacitance element (see U.S. Pat. No. 6,593,672, for example). The digital variable-capacitance device is constituted such that n (≧1) variable-capacitance elements are connected in parallel and the i (i=1, 2, . . . , n)th variable-capacitance element takes only binary of 0 [pF] or 2iC [pF]. Therefore, a variable-capacitance device can be obtained that is provided with a variable range of 0[pF] to (2n+1−2) C [pF] and a step size of a capacitance C [pF] as a whole by controlling the variable-capacitance elements connected in parallel in a binary manner.
In the digital variable-capacitance element, a variable capacitance having binary of 0 [pF] and 2iC [pH] (i=1, 2, . . . , n) by changing a distance between electrodes using an actuator is produced. As the actuator, there are ones of a static type, a thermal type, a electromagnetic type, a piezoelectric type, and so on. In such a digital variable-capacitance element, there are two factors important for improving performance as follows:
1) A resistance is small (Q value is large)
2) A capacitance ratio is large
Here, the term “capacitance ratio” means a ratio Cmax/Cmin of a capacitance value obtained when a distance between electrodes is the minimum (hereinafter, “Cmax) to a capacitance value obtained when the distance between electrodes is the maximum (hereinafter, “Cmin”). When the distance between electrodes is made sufficiently large, the capacitance value Cmin can be caused to approach to 0 [pF], but the distance between electrodes is at most about several micron meters in an ordinary digital variable-capacitance element manufactured using the MEMS technology. Consequently, a capacitance ratio of several tens can be only achieved in a conventional digital variable-capacitance element. When the capacitance ratio is small, there occurs such a problem that, when digital capacitance elements are arranged in parallel, as described in U.S. Pat. No. 6,593,672, a variable width can not be increased due to contribution of the Cmin.